This invention relates generally to semiconductor switching devices such as thyristors and more particularly to a method and apparatus for maintaining matched time of turn off for gate turn-off type thyristors operated in parallel between two electrical power sources and adapted to transfer electrical power between the two sources.
There are many applications where, because of current ratings, two or more switching devices are connected in parallel and controlled so that they are simultaneously conductive to transfer electrical power between a source and a load, the latter in certain applications also representing a source during certain phases of circuit operation. When the switching devices are of the non-proportional, semiconductor type, particularly thyristors including thyristors of the gate turn-off (GTO) type, it is desired not only to maintain current balance between the several parallely connected devices as shown and described in the above-referenced related application, U.S. Ser. No. 07/382,942, but also to have these devices become non-conductive simultaneously.
Accordingly, where there are several thyristors, hereinafter referred to simply as GTOs, which are connected in parallel and therefore carrying similar currents, are all given a signal to turn off, i.e. cease conduction at the same instant, the actual times of turn-off will be determined by the devices themselves. If the GTOs are perfectly matched, all of the devices will turn off concurrently; however, if otherwise, the first GTOs to turn off will tend to transfer their respective current to the remaining GTOs, with the result being that the last GTO to turn off is carrying substantially all of the current carried by the other GTOs which have since turned off. The amount of current carried by the last conducting GTO is a function of the turn-off times of the several GTOs and the effect of their respective snubber circuits.
Accordingly, prior art practice has either ignored the problem because time of turn off balancing control is unnecessary due to the relatively low current applications, or one must resort to extensive pretesting of whole groups of GTOs intended for use and thereafter using matched sets which have substantially the same turn-off times.